2 * Digital Signature Standard implementation for PuTTY.
12 static void sha_mpint(SHA_State * s, Bignum b)
14 unsigned char lenbuf[4];
16 len = (bignum_bitcount(b) + 8) / 8;
17 PUT_32BIT(lenbuf, len);
18 SHA_Bytes(s, lenbuf, 4);
20 lenbuf[0] = bignum_byte(b, len);
21 SHA_Bytes(s, lenbuf, 1);
23 smemclr(lenbuf, sizeof(lenbuf));
26 static void sha512_mpint(SHA512_State * s, Bignum b)
28 unsigned char lenbuf[4];
30 len = (bignum_bitcount(b) + 8) / 8;
31 PUT_32BIT(lenbuf, len);
32 SHA512_Bytes(s, lenbuf, 4);
34 lenbuf[0] = bignum_byte(b, len);
35 SHA512_Bytes(s, lenbuf, 1);
37 smemclr(lenbuf, sizeof(lenbuf));
40 static void getstring(char **data, int *datalen, char **p, int *length)
45 *length = toint(GET_32BIT(*data));
50 if (*datalen < *length)
56 static Bignum getmp(char **data, int *datalen)
62 getstring(data, datalen, &p, &length);
66 return NULL; /* negative mp */
67 b = bignum_from_bytes((unsigned char *)p, length);
71 static Bignum get160(char **data, int *datalen)
78 b = bignum_from_bytes((unsigned char *)*data, 20);
85 static void dss_freekey(void *key); /* forward reference */
87 static void *dss_newkey(char *data, int len)
93 dss = snew(struct dss_key);
94 getstring(&data, &len, &p, &slen);
100 for (i = 0; i < len; i++)
101 printf(" %02x", (unsigned char) (data[i]));
106 if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
110 dss->p = getmp(&data, &len);
111 dss->q = getmp(&data, &len);
112 dss->g = getmp(&data, &len);
113 dss->y = getmp(&data, &len);
116 if (!dss->p || !dss->q || !dss->g || !dss->y ||
117 !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
126 static void dss_freekey(void *key)
128 struct dss_key *dss = (struct dss_key *) key;
142 static char *dss_fmtkey(void *key)
144 struct dss_key *dss = (struct dss_key *) key;
146 int len, i, pos, nibbles;
147 static const char hex[] = "0123456789abcdef";
150 len = 8 + 4 + 1; /* 4 x "0x", punctuation, \0 */
151 len += 4 * (bignum_bitcount(dss->p) + 15) / 16;
152 len += 4 * (bignum_bitcount(dss->q) + 15) / 16;
153 len += 4 * (bignum_bitcount(dss->g) + 15) / 16;
154 len += 4 * (bignum_bitcount(dss->y) + 15) / 16;
155 p = snewn(len, char);
160 pos += sprintf(p + pos, "0x");
161 nibbles = (3 + bignum_bitcount(dss->p)) / 4;
164 for (i = nibbles; i--;)
166 hex[(bignum_byte(dss->p, i / 2) >> (4 * (i % 2))) & 0xF];
167 pos += sprintf(p + pos, ",0x");
168 nibbles = (3 + bignum_bitcount(dss->q)) / 4;
171 for (i = nibbles; i--;)
173 hex[(bignum_byte(dss->q, i / 2) >> (4 * (i % 2))) & 0xF];
174 pos += sprintf(p + pos, ",0x");
175 nibbles = (3 + bignum_bitcount(dss->g)) / 4;
178 for (i = nibbles; i--;)
180 hex[(bignum_byte(dss->g, i / 2) >> (4 * (i % 2))) & 0xF];
181 pos += sprintf(p + pos, ",0x");
182 nibbles = (3 + bignum_bitcount(dss->y)) / 4;
185 for (i = nibbles; i--;)
187 hex[(bignum_byte(dss->y, i / 2) >> (4 * (i % 2))) & 0xF];
192 static char *dss_fingerprint(void *key)
194 struct dss_key *dss = (struct dss_key *) key;
195 struct MD5Context md5c;
196 unsigned char digest[16], lenbuf[4];
197 char buffer[16 * 3 + 40];
202 MD5Update(&md5c, (unsigned char *)"\0\0\0\7ssh-dss", 11);
204 #define ADD_BIGNUM(bignum) \
205 numlen = (bignum_bitcount(bignum)+8)/8; \
206 PUT_32BIT(lenbuf, numlen); MD5Update(&md5c, lenbuf, 4); \
207 for (i = numlen; i-- ;) { \
208 unsigned char c = bignum_byte(bignum, i); \
209 MD5Update(&md5c, &c, 1); \
217 MD5Final(digest, &md5c);
219 sprintf(buffer, "ssh-dss %d ", bignum_bitcount(dss->p));
220 for (i = 0; i < 16; i++)
221 sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
223 ret = snewn(strlen(buffer) + 1, char);
229 static int dss_verifysig(void *key, char *sig, int siglen,
230 char *data, int datalen)
232 struct dss_key *dss = (struct dss_key *) key;
236 Bignum r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v;
246 for (i = 0; i < siglen; i++)
247 printf(" %02x", (unsigned char) (sig[i]));
252 * Commercial SSH (2.0.13) and OpenSSH disagree over the format
253 * of a DSA signature. OpenSSH is in line with RFC 4253:
254 * it uses a string "ssh-dss", followed by a 40-byte string
255 * containing two 160-bit integers end-to-end. Commercial SSH
256 * can't be bothered with the header bit, and considers a DSA
257 * signature blob to be _just_ the 40-byte string containing
258 * the two 160-bit integers. We tell them apart by measuring
259 * the length: length 40 means the commercial-SSH bug, anything
260 * else is assumed to be RFC-compliant.
262 if (siglen != 40) { /* bug not present; read admin fields */
263 getstring(&sig, &siglen, &p, &slen);
264 if (!p || slen != 7 || memcmp(p, "ssh-dss", 7)) {
267 sig += 4, siglen -= 4; /* skip yet another length field */
269 r = get160(&sig, &siglen);
270 s = get160(&sig, &siglen);
279 if (!bignum_cmp(s, Zero)) {
286 * Step 1. w <- s^-1 mod q.
288 w = modinv(s, dss->q);
296 * Step 2. u1 <- SHA(message) * w mod q.
298 SHA_Simple(data, datalen, (unsigned char *)hash);
301 sha = get160(&p, &slen);
302 u1 = modmul(sha, w, dss->q);
305 * Step 3. u2 <- r * w mod q.
307 u2 = modmul(r, w, dss->q);
310 * Step 4. v <- (g^u1 * y^u2 mod p) mod q.
312 gu1p = modpow(dss->g, u1, dss->p);
313 yu2p = modpow(dss->y, u2, dss->p);
314 gu1yu2p = modmul(gu1p, yu2p, dss->p);
315 v = modmul(gu1yu2p, One, dss->q);
318 * Step 5. v should now be equal to r.
321 ret = !bignum_cmp(v, r);
337 static unsigned char *dss_public_blob(void *key, int *len)
339 struct dss_key *dss = (struct dss_key *) key;
340 int plen, qlen, glen, ylen, bloblen;
342 unsigned char *blob, *p;
344 plen = (bignum_bitcount(dss->p) + 8) / 8;
345 qlen = (bignum_bitcount(dss->q) + 8) / 8;
346 glen = (bignum_bitcount(dss->g) + 8) / 8;
347 ylen = (bignum_bitcount(dss->y) + 8) / 8;
350 * string "ssh-dss", mpint p, mpint q, mpint g, mpint y. Total
351 * 27 + sum of lengths. (five length fields, 20+7=27).
353 bloblen = 27 + plen + qlen + glen + ylen;
354 blob = snewn(bloblen, unsigned char);
358 memcpy(p, "ssh-dss", 7);
363 *p++ = bignum_byte(dss->p, i);
367 *p++ = bignum_byte(dss->q, i);
371 *p++ = bignum_byte(dss->g, i);
375 *p++ = bignum_byte(dss->y, i);
376 assert(p == blob + bloblen);
381 static unsigned char *dss_private_blob(void *key, int *len)
383 struct dss_key *dss = (struct dss_key *) key;
386 unsigned char *blob, *p;
388 xlen = (bignum_bitcount(dss->x) + 8) / 8;
391 * mpint x, string[20] the SHA of p||q||g. Total 4 + xlen.
394 blob = snewn(bloblen, unsigned char);
399 *p++ = bignum_byte(dss->x, i);
400 assert(p == blob + bloblen);
405 static void *dss_createkey(unsigned char *pub_blob, int pub_len,
406 unsigned char *priv_blob, int priv_len)
409 char *pb = (char *) priv_blob;
413 unsigned char digest[20];
416 dss = dss_newkey((char *) pub_blob, pub_len);
419 dss->x = getmp(&pb, &priv_len);
426 * Check the obsolete hash in the old DSS key format.
429 getstring(&pb, &priv_len, &hash, &hashlen);
432 sha_mpint(&s, dss->p);
433 sha_mpint(&s, dss->q);
434 sha_mpint(&s, dss->g);
435 SHA_Final(&s, digest);
436 if (0 != memcmp(hash, digest, 20)) {
443 * Now ensure g^x mod p really is y.
445 ytest = modpow(dss->g, dss->x, dss->p);
446 if (0 != bignum_cmp(ytest, dss->y)) {
456 static void *dss_openssh_createkey(unsigned char **blob, int *len)
458 char **b = (char **) blob;
461 dss = snew(struct dss_key);
463 dss->p = getmp(b, len);
464 dss->q = getmp(b, len);
465 dss->g = getmp(b, len);
466 dss->y = getmp(b, len);
467 dss->x = getmp(b, len);
469 if (!dss->p || !dss->q || !dss->g || !dss->y || !dss->x ||
470 !bignum_cmp(dss->q, Zero) || !bignum_cmp(dss->p, Zero)) {
479 static int dss_openssh_fmtkey(void *key, unsigned char *blob, int len)
481 struct dss_key *dss = (struct dss_key *) key;
485 ssh2_bignum_length(dss->p) +
486 ssh2_bignum_length(dss->q) +
487 ssh2_bignum_length(dss->g) +
488 ssh2_bignum_length(dss->y) +
489 ssh2_bignum_length(dss->x);
496 PUT_32BIT(blob+bloblen, ssh2_bignum_length((x))-4); bloblen += 4; \
497 for (i = ssh2_bignum_length((x))-4; i-- ;) blob[bloblen++]=bignum_byte((x),i);
507 static int dss_pubkey_bits(void *blob, int len)
512 dss = dss_newkey((char *) blob, len);
515 ret = bignum_bitcount(dss->p);
521 static unsigned char *dss_sign(void *key, char *data, int datalen, int *siglen)
524 * The basic DSS signing algorithm is:
526 * - invent a random k between 1 and q-1 (exclusive).
527 * - Compute r = (g^k mod p) mod q.
528 * - Compute s = k^-1 * (hash + x*r) mod q.
530 * This has the dangerous properties that:
532 * - if an attacker in possession of the public key _and_ the
533 * signature (for example, the host you just authenticated
534 * to) can guess your k, he can reverse the computation of s
535 * and work out x = r^-1 * (s*k - hash) mod q. That is, he
536 * can deduce the private half of your key, and masquerade
537 * as you for as long as the key is still valid.
539 * - since r is a function purely of k and the public key, if
540 * the attacker only has a _range of possibilities_ for k
541 * it's easy for him to work through them all and check each
542 * one against r; he'll never be unsure of whether he's got
545 * - if you ever sign two different hashes with the same k, it
546 * will be immediately obvious because the two signatures
547 * will have the same r, and moreover an attacker in
548 * possession of both signatures (and the public key of
549 * course) can compute k = (hash1-hash2) * (s1-s2)^-1 mod q,
550 * and from there deduce x as before.
552 * - the Bleichenbacher attack on DSA makes use of methods of
553 * generating k which are significantly non-uniformly
554 * distributed; in particular, generating a 160-bit random
555 * number and reducing it mod q is right out.
557 * For this reason we must be pretty careful about how we
558 * generate our k. Since this code runs on Windows, with no
559 * particularly good system entropy sources, we can't trust our
560 * RNG itself to produce properly unpredictable data. Hence, we
561 * use a totally different scheme instead.
563 * What we do is to take a SHA-512 (_big_) hash of the private
564 * key x, and then feed this into another SHA-512 hash that
565 * also includes the message hash being signed. That is:
567 * proto_k = SHA512 ( SHA512(x) || SHA160(message) )
569 * This number is 512 bits long, so reducing it mod q won't be
570 * noticeably non-uniform. So
574 * This has the interesting property that it's _deterministic_:
575 * signing the same hash twice with the same key yields the
578 * Despite this determinism, it's still not predictable to an
579 * attacker, because in order to repeat the SHA-512
580 * construction that created it, the attacker would have to
581 * know the private key value x - and by assumption he doesn't,
582 * because if he knew that he wouldn't be attacking k!
584 * (This trick doesn't, _per se_, protect against reuse of k.
585 * Reuse of k is left to chance; all it does is prevent
586 * _excessively high_ chances of reuse of k due to entropy
589 * Thanks to Colin Plumb for the general idea of using x to
590 * ensure k is hard to guess, and to the Cambridge University
591 * Computer Security Group for helping to argue out all the
594 struct dss_key *dss = (struct dss_key *) key;
596 unsigned char digest[20], digest512[64];
597 Bignum proto_k, k, gkp, hash, kinv, hxr, r, s;
598 unsigned char *bytes;
601 SHA_Simple(data, datalen, digest);
604 * Hash some identifying text plus x.
607 SHA512_Bytes(&ss, "DSA deterministic k generator", 30);
608 sha512_mpint(&ss, dss->x);
609 SHA512_Final(&ss, digest512);
612 * Now hash that digest plus the message hash.
615 SHA512_Bytes(&ss, digest512, sizeof(digest512));
616 SHA512_Bytes(&ss, digest, sizeof(digest));
619 SHA512_State ss2 = ss; /* structure copy */
620 SHA512_Final(&ss2, digest512);
622 smemclr(&ss2, sizeof(ss2));
625 * Now convert the result into a bignum, and reduce it mod q.
627 proto_k = bignum_from_bytes(digest512, 64);
628 k = bigmod(proto_k, dss->q);
630 kinv = modinv(k, dss->q); /* k^-1 mod q */
631 if (!kinv) { /* very unlikely */
633 /* Perturb the hash to think of a different k. */
634 SHA512_Bytes(&ss, "x", 1);
635 /* Go round and try again. */
642 smemclr(&ss, sizeof(ss));
644 smemclr(digest512, sizeof(digest512));
647 * Now we have k, so just go ahead and compute the signature.
649 gkp = modpow(dss->g, k, dss->p); /* g^k mod p */
650 r = bigmod(gkp, dss->q); /* r = (g^k mod p) mod q */
653 hash = bignum_from_bytes(digest, 20);
654 hxr = bigmuladd(dss->x, r, hash); /* hash + x*r */
655 s = modmul(kinv, hxr, dss->q); /* s = k^-1 * (hash + x*r) mod q */
665 * string two 20-byte numbers r and s, end to end
667 * i.e. 4+7 + 4+40 bytes.
669 nbytes = 4 + 7 + 4 + 40;
670 bytes = snewn(nbytes, unsigned char);
672 memcpy(bytes + 4, "ssh-dss", 7);
673 PUT_32BIT(bytes + 4 + 7, 40);
674 for (i = 0; i < 20; i++) {
675 bytes[4 + 7 + 4 + i] = bignum_byte(r, 19 - i);
676 bytes[4 + 7 + 4 + 20 + i] = bignum_byte(s, 19 - i);
685 const struct ssh_signkey ssh_dss = {
692 dss_openssh_createkey,